Not Applicable
1. Field of the Invention
The present invention generally relates to the manufacture of solid state polymer electrolytes. More particularly, the invention relates to highly conductive thin lithium polymer electrolyte structures and the methods by which they are made. The invention also relates to methods of manufacturing all-solid-state electrochemical cells based on such electrolytes.
2. Description of the Related Art
Throughout the world there are ongoing efforts to develop an all solid state, rechargeable, high energy density battery using a thin polymer film as the electrolyte. Since the concept of such a battery, based on the use of polyethylene oxide/lithium salt complexes, was first discussed in 1979 by Armand et al. in Fast Ion Transport in Solids (eds. P. Vashista et al., North-Holland, Amsterdam, p.131), development has mainly centered around rechargeable systems utilizing intercalation/insertion compounds. Some of the more recent work has focused on designing novel polymers with higher ionic conductivities at ambient temperature. Solid state battery and capacitor technology, including the evolution of polymer electrolytes, is discussed in the Handbook of Solid State Batteries and Capacitors, M. Z. A. Munshi, ed., World Scientific Publishing, Inc., Singapore, which is incorporated herein by reference.
At the present time, the state-of-the-art lithium battery is a lithium ion battery which uses a carbon electrode as the negative electrode or anode and a lithiated metal oxide, such as lithiated cobalt oxide, lithiated nickel oxide, lithiated manganese oxide, or mixtures of these materials as the positive electrode or cathode, a microporous polypropylene or polyethylene separator that separates the two electrodes and prevents them from shorting electrically, and liquid organic solvents containing a lithium salt as the electrolyte. The electrolyte is usually absorbed into the separator material and provides high ionic conductivity (10xe2x88x923 to 10xe2x88x922 S/cm) and migration of ions between the electrodes of the cell. An offshoot of the lithium ion battery is the lithium ion polymer electrolyte battery. The electrode chemistry of this battery is the same; however in this case the liquid electrolyte (up to 70% by weight of the electrolyte) is absorbed in a polymer membrane instead of the microporous polypropylene separator.
Another type of rechargeable lithium battery system sometimes used today employs a lithium metal anode. Secondary batteries using lithium metal as the negative electrode, intercalation or insertion compounds as the positive electrode, and non-aqueous organic electrolytes were the focus of intense investigation during the 1970""s and 1980""s. One problem, however, with using lithium in a rechargeable system is that because of the instability of lithium in these solvents, a large excess of lithium is required to off-set the chemical reaction of lithium with the solvent, usually as much as 3 to 5 times more lithium capacity than the cathode capacity. In addition, the liquid solvent electrolyte employed in any of the above-described cell systems is often corrosive and toxic, and presents handling difficulties due to spillage or leakage from the cell. Liquid solvent electrolyte can also release gas, or outgas, during overcharge or overdischarge or at elevated temperatures, leading to safety problems.
In order to overcome the disadvantages inherent in liquid electrolytes and to obtain better long-term storage stability there is interest in developing solid polymeric electrolytes in which ion mobility is achieved through coordination by sites on the polymer chain of electrolyte ions, thus promoting electrolyte dissolution and salt dissociation. An all-solid-state battery using an ionically conductive polymer membrane as the electrolyte would have several attractive features. It could be made into virtually any shape and size, be reasonably rugged and leakproof, and have low self-discharge. It could be made into thin film power cells or thick film energy cells, would have high open-circuit potentials using a lithium anode, and could be produced by automated fabrication techniques. These features represent a unique combination of properties and give rise to the possibility of using such batteries, as either secondary or primary devices across a wide range of applications.
In an attempt to develop all-solid-state polymer electrolyte, one polymer that has been examined extensively is polyethylene oxide (PEO), which is able to form stable complexes with a number of salts. Because of its low ionic conductivity at ambient temperature of approximately 10xe2x88x929 to 10xe2x88x928 S/cm, batteries examined using this material had to operate at 100xc2x0 C. and above. A major problem with PEO based electrolytes at temperatures below 60xc2x0 C. is their high crystallinity and the associated low ion mobility. In recent years a number of radically different approaches have been taken to improve the conductivity of PEO and PEO-based polymers that have also led to the proposal of other polymers. These approaches included polymer modifications and synthesizing new polymers; forming composite polymers with ceramic materials; using plasticizer salts to increase the ion transport and mobility of the cation; using plasticizing solvents in the polymer again to increase the ionic character of the cation; among other approaches. Several review articles describe these approaches in detail, e.g. xe2x80x9cTechnology Assessment of Lithium Polymer Electrolyte Secondary Batteriesxe2x80x9d by M. Z. A. Munshi, Chapter 19 in Handbook of Solid State State Batteries and Capacitors, Ed. M. Z. A. Munshi (World Scientific Pub. Singapore) 1995; A. Hooper, M. Gauthier, and A. Belanger, in: xe2x80x9cElectrochemical Science and Technology of Polymersxe2x80x942, Ed. R. G. Linford (Elsevier Applied Science, London), 1987.
Polymer modification and synthesis of new polymers resulted in some improvement in the ionic conductivity but the mechanical property and integrity were poor. Probably, the best known polymer as a result of this synthesis is poly(bis(methoxyethoxyethoxide))-phosphazene, known as MEEP, which has an ionic conductivity of approximately 10xe2x88x925 S/cm at room temperature when combined with a lithium salt, but which has glue-like mechanical properties. On the other hand, materials based on blocked copolymers may provide alternatives. For example, PEO-PPO-PEO crosslinked with trifunctional urethane and a lithium salt has an ionic conductivity of approximately 10xe2x88x925 S/cm but is too rigid, brittle and difficult to manufacture.
Inorganic conducting and non-conducting fillers have also been used to increase the ionic conductivity and mechanical property of the polymer. Addition of alpha alumina to (PEO)8.LiClO4 in resulted in a negligible effect on the ionic conductivity but dramatically increased the mechanical property at 100xc2x0 C., while the addition of other ceramic materials such as ionically conductive beta alumina to PEO-NaI and PEO-LiClO4 complexes improved the ionic conductivity of PEO based electrolytes to approximately 10xe2x88x925 S/cm. In another battery technology, inorganic fillers based on high surface area alumina and silica have been used to enhance the ionic conductivity of lithium iodide solid electrolyte from 10xe2x88x927 S/cm to 10xe2x88x925-10xe2x88x924 S/cm at room temperature (see C. C. Liang, J. Electrochemical Society, Vol. 120, page 1289 (1973)). Plasticizer salts based on lithium. bis(trifluoromethane sulfonyl)imide, LiN(CF3SO2)2 trademarked as LiTFSI by Hydro-Quebec and distributed by the 3M Company under the product name, HQ-115 when added to PEO yields a conductivity of about 10xe2x88x925 S/cm.
None of the previous approaches toward improving polymer conductivity has resulted in adequate conductivity enhancements of the polymer electrolytes to permit room temperature operation of batteries utilizing the electrolyte. Accordingly, an attempt was made to increase the ionic conductivity of PEO-based polymer electrolyte by incorporating plasticizing solvents or low molecular weight polymers to the polymer electrolyte. The intent was to increase the ionic mobility and concentrations of the charge carriers in the solid polymer electrolyte by enhancing the dissociation of the lithium salt. Generally, it is believed that the lithium ion is also solvated to the solvent molecule and participates in enhancing the ionic mobility. Many electrolyte composites incorporating low molecular weight polymers or liquid organic solvents have been prepared and have demonstrated high room temperature conductivity approaching those of the typical non-aqueous liquid electrolytes. For example, Kelly et al. (J Power Sources, 14:13 (1985)) demonstrate that adding 20 mole percent of liquid polyethylene glycol dimethyl ether polymer (PEGDME) to solid PEO polymer results in an increase in the ionic conductivity of the final plasticized polymer from 3xc3x9710xe2x88x927 S/cm to 10xe2x88x924 S/cm at 40xc2x0 C. However, the mechanical property of this material was very poor.
Bauer et al in U.S. Pat. No. 4,654,279 (1987) demonstrate that thermal crosslinking of polymers consisting of epoxies and methacrylates and plasticized with a solution of LiClO4 in a 400 MW PEG resulted in a conductivity of 4xc3x9710xe2x88x924 S/cm at 25xc2x0 C. This patent describes a polymeric electrolyte consisting of a two phase interpenetrating network (IPN) of a mechanically supporting phase of a continuous network of a cross-linked polymer and an ionically conducting phase comprising of a metal salt and a liquid polymer such as liquid PEG.
Many of these low molecular weight polymers have a relatively low dielectric constant when compared to their liquid solvent counterpart, and thus limit the number of charge carriers in the plasticized polymer. In an effort to overcome this hindrance, high dielectric constant liquid organic solvent such as ethylene carbonate (EC) and propylene carbonate (PC) have been incorporated in the host polymer both to increase the number of charge carriers and increase further the room temperature conductivity of the polymer. The use of these organic solvents to plasticize polymers such as poly(vinyl acetal), poly(acrylonitrile), poly(vinyl acetate) and hexafluoropropenevinylidene fluoride copolymer (Viton(trademark)) were made as early as 1975 by Feuillade and Perche (Journal of Applied Electrochemistry, Vol. 5, page 63 (1975)). However, the mechanical properties of these polymers were so poor that they had to be supported on porous matrices. Later Armand (Proc. Workshop on Li Non-Aqueous Battery Electrochemistry, The Electrochemical Soc. Vol. 80-7, page 261 (1980)) produced a system with good room temperature conductivity (10xe2x88x924 S/cm) and good mechanical properties by crosslinking Viton(trademark) and plasticizing with a solution of 1M LiClO4 in PC. Polyvinylidene fluoride (PVDF) and polyacrylonitrile (PAN) were evaluated in the early 1980s and have also been doped with a variety of liquid polar solvents, yielding room temperature conductivities as high as 10xe2x88x923 S/cm. Subsequently, PVDF has been the subject of a recent patent from Bellcore (U.S. Pat. No. 5,296,318).
The use of PC in an ionically conductive matrix containing oxygen donor atoms such as PEO complexed with a lithium salt was first presented by the present inventor in a paper presented at the Fall Meeting of the Electrochemical Soc. held Oct. 18-23, 1987. Although room temperature battery performance data was presented at that time, the propylene carbonate/lithium salt/polymer electrolyte did not have good mechanical properties. In the late 1980""s through early 1990""s, a series of U.S patents including U.S. Pat. Nos. 4,792,504; 4,747,542; 4,792,504; 4,794,059; 4,808,496; 4,816,357; 4,830,939; 4,861,690; 4,925,751; 4,925,752; 4,935,317; 4,960,655; 4,990,413; 4,997,732; 5,006,431; 5,030,527; 5,057,385; 5,066,554 and European patents EP 0 359 524 and EP 0 411 949 were issued variously to MHB Inc. and Hope Industries. These patents generally relate to the use of liquid organic solvents in various types of polymeric materials including PEO, materials based on acrylates, and low MW PEG acrylates. These patents described predominantly radiation curing methods for the preparation of interpenetrating polymeric networks containing various types of polyacrylates and liquid organic solvents. Although electron beam curing was the preferred method for polymerizing the IPN, thermal and ultraviolet curing methods were also proposed. The idea behind this was to contain the PC solution in the matrix of the polymeric network that would therefore yield a high ionic conductivity comparable to that of PC itself. Indeed, this was demonstrated in typical polymeric networks, yielding conductivities of about 2xc3x9710xe2x88x923 S/cm at room temperature. An advantage with using electron beam curing compared to UV radiation is that an electron beam can penetrate through metallic components, and hence complete prototype cells can be made in-situ.
While the addition of organic plasticizers may help solver the problem of low ionic conductivity in polymer electrolytes, they necessarily introduce additional electrolyte components that may have deleterious effects on other electrolyte properties, such as stability in contact with metallic lithium. Like the liquid organic electrolytes, plasticized polymer electrolyte is not thermodynamically stable at the lithium metal potential. In addition, polymer electrolytes based on such designs cannot be manufactured in very thin film form so as to reduce their overall resistance and hence cell resistance, since the polymer will not have the sufficient strength to hold the liquid organic solvents in its matrix. For such a system to be fully functional it must be based on a thick film concept, which increases its overall cell resistance and reduces the energy density due to a reduction in the active components in the cell. Another problem with this type of design is the fact that polymers containing liquids cannot be wound along with the rest of the electrode components in a winding machine, since the liquid will tend to ooze out of the polymer as soon as any stress is applied to the polymer.
All of the prior art techniques that have been employed to improve the ionic conductivity, mechanical strength, safety, and chemical stability, and to reduce cost by simplifying or improving the synthesis of polymer electrolytes have serious shortcomings. As a result, there is, still no room temperature conducting polymer electrolyte available today that is entirely suitable for use with a lithium metal anode in a rechargeable lithium battery. There remains a need for solid polymer electrolytes with better ionic conductivity at room temperature and below so that the performance of an electrochemical cell at room temperature, or below room temperature, can be improved. There is also a great need for thinner components for batteries that avoid the use of organic solvents in the electrolyte without sacrificing energy density.
The solid polymer electrolytes and electrochemical cells of the preferred embodiments of the present invention solve many of the problems and disadvantages described above. The all-solid-state polymer electrolytes offer distinct advantages over prior polymer electrolytes. Some of these advantages include an all-solid-state composition containing no liquid organic solvents, and characterized by high stability with increased temperature. These electrolytes are also mechanically stronger than previous polymer electrolyte compositions, and for the first time provide a commercially feasible solid polymer electrolyte with good ionic conductivity at room temperature and below. Another advantage of many of the new polymer electrolyte compositions of the invention is that they can be manufactured in ultra-thin film form, are non-tacky, are pin-hole free, and provide low resistance and excellent conformal and flexible design capability. A particular advantage of certain solid lithium polymer electrolyte compositions of the invention is chemical stability, which permits their use with lithium metal as the anode of a polymer electrolyte battery. Lithium polymer electrolyte batteries constructed from the new polymer electrolyte compositions can be used in any orientation, a quality that is not possible today with conventional lithium batteries. Still another advantage of the preferred all-solid-state polymer electrolytes of the invention is that as a result of their solid state they are tolerant to overcharge when used in a solid state lithium polymer electrolyte battery. Since there is no liquid solvent within the solid polymer electrolyte, overcharge of batteries using the new solid polymeric electrolytes does not involve any gassing reactions. Instead the polymer merely degrades to a crystalline deposit. Hence, batteries constructed from such solid polymer electrolytes are safer than their liquid electrolyte counterparts.
In accordance with the present invention, a solid polymer electrolyte having a conductivity greater than 1xc3x9710xe2x88x924 S/cm at 25xc2x0 C. or below is provided. The solid polymer electrolytes comprise a mixture of a base polymer material comprising at least one ionically conductive polymer and having an initial conductivity of at least about 1xc3x9710xe2x88x925 S/cm at 25xc2x0 C. when combined with a metal salt; a metal salt, an inorganic filler having an average particle size less than 0.05 micron in diameter and a surface area of at least about 100 m2/g; and an ionic conducting material having an average particle size less than 0.1 micron in diameter and an initial ionic conductivity of at least 2xc3x9710xe2x88x923 S/cm at 25xc2x0 C. Preferably the electrolyte is a cationic conductor or has cationic mobility, and the metal salt is a sodium, lithium, potassium, magnesium or calcium salt, more preferably lithium. In certain embodiments, the salt is a lithium salt. Preferred lithium salts include lithium hexafluorophosphate (LiPF6), lithium perchlorate (LiClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluoroarsenate (LiAsF6), lithium tetrachloroaluminate (LiAlCl4), lithium trifluoromethanesulfonate (LiCF3SO3), lithium methide (LiC(SO2CF3)3 and lithium bis(trifluoromethane sulfonyl)imide (LiN(CF3SO2)2 or lithium imide). It is preferred that the lithium salt is a plasticizer salt.
Some embodiments of the solid polymer electrolyte comprise about 30 to 95% (by weight of solid polymer electrolyte) of the base polymer material; about 1 to 25% (by weight of solid polymer electrolyte) of the metal salt; about 0.1-60% (by volume of solid polymer electrolyte) of the inorganic filler; and about 0.1-60% (by volume of solid polymer electrolyte) of the ionic conducting material. In some of the preferred embodiments, the amount of the inorganic filler is about 0.1-20% (by volume of solid polymer electrolyte), and the concentration of the ionic conducting material is about 0.1-20% (by volume of solid polymer electrolyte).
In certain embodiments of the solid polymer electrolytes the metal salt is a plasticizer lithium salt, the inorganic filler is fumed silica or alumina, and a glassy lithium ion conductor or a ceramic lithium ion conductor is employed. The lithium ion conducting material may be a sulfide glass, lithium beta alumina, a lithium silicate, lithium phosphorus oxynitride (Li3PO4), or another phosphate glass in some of the more preferred embodiments of the solid polymer electrolytes.
In some embodiments the base polymer material comprises at least two polymers, the first of which is an ionically conductive polymer. An ionically conductive polymer is one in which the monomers contain a hetero atom with a lone pair of electrons available for the metal ions of a metal salt to attach to and move between during conduction, when the polymer is mixed with a metal salt. In such a polymer, movement of the metal ion from one lone electron pair site to another during the conduction process is facilitated. The first polymer is a linear polymer, a random copolymer, a block copolymer, a comb-branched block copolymer, a network structure, a single ion conductor, a polyvinylidene fluoride or chloride or copolymer of their derivatives, poly(chlorotrifluoroethylene), poly(ethylene-chlorotrifluoro-ethylene), or poly(fluorinated ethylene-propylene) in some of the preferred embodiments of the solid polymer electrolytes. In certain embodiments, the first polymer is chosen from the group consisting of polyethylene oxide (PEO), oxymethylene linked PEO, PEO-PPO-PEO crosslinked with trifunctional urethane; poly(bis(methoxy-ethoxy-ethoxide))-phosphazene (MEEP); a triol-type PEO crosslinked with difunctional urethane, poly((oligo)oxyethylene)methacrylate-co-alkali metal methacrylate; polyacrylonitrile (PAN), polymethylmethacrylate (PNMA), polymethylacrylonitrile (PMAN); polysiloxanes and their copolymers and derivatives, polyvinylidene fluoride or chloride and copolymers of their derivatives, poly(chlorotrifluoro-ethylene), poly(ethylene-chlorotrifluoroethylene), poly(fluorinated ethylene-propylene), acrylate-based polymer, other similar solvent-free polymers, combinations of the foregoing polymers either condensed or crosslinked to form a different polymer, and physical mixtures of any of the foregoing polymers, provided that the polymer or polymer mixture is combinable with a lithium salt such that the ionic conductivity of the first polymer is enhanced compared to its conductivity when not combined with the lithium salt.
In some embodiments of the solid polymer electrolyte the second polymer is more inert with respect to ionic conductivity and is stronger than the first polymer when each polymer is in the form of a thin film. Some suitable polymers for the second polymer include polyester (PET), polypropylene (PP), polyethylene napthalate (PEN), polyvinylidene fluoride (PVDF), polycarbonate (PC), polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), or another polymer that has thermodynamic stability and strength characteristics similar to one of the foregoing polymers.
Certain of the solid polymer electrolytes contain a base polymer material comprising about 1% to 99% (by weight of base polymer material) of one polymer and the remainder of the base polymer material comprises at least one other polymer.
In some embodiments, the base polymer material of the solid polymer electrolyte is dimensionally stable up to at least 150xc2x0 C. Some embodiments of the solid polymer electrolyte include a polymer with a non-linear molecular-structure and the electrolyte has a resilient, amorphous structure, providing a mechanically stronger product than prior art polymer electrolyte compositions. The solid polymer electrolyte may be in the form of a 0.2 to 100 micron thick film, preferably 0.2 to 10 micron, and more preferably 0.2 to 3 micron thick. Such ultra-thin polymer electrolyte films are characterized by a resistance of no more than about 1 xcexa9/cm2 when employed as a thin-film electrolyte in an electrochemical cell.
One chemically stable solid polymer electrolyte especially suited for use with a lithium metal anode of a polymer electrolyte battery comprises about 30 to 95% (by weight of solid polymer electrolyte) base polymer material comprising at least one ionically conductive polymer and having an initial conductivity of at least about 10xe2x88x925 S/cm at 25xc2x0 C. when combined with a lithium salt; about 1 to 25% (by weight of the solid polymer electrolyte) lithium salt; about 0.1-60% (by volume of the solid polymer electrolyte) inorganic filler having an average particle size less than 0.05 micron in diameter and a surface area of at least about 100 m2/g; and about 0.1-80% (by volume of the solid polymer electrolyte) lithium ion conducting material having an average particle size less than 0.1 micron in diameter and an initial ionic conductivity of at least 2xc3x9710xe2x88x923 S/cm at 25xc2x0 C. All solid state lithium-based cells employing this electrolyte use less lithium in the cell than conventional lithium ion cells, therefore reducing cost and increasing energy content.
Preferred base polymer materials for a solid polymer electrolyte have a conductivity of at least about 5xc3x9710xe2x88x925 S/cm at 25xc2x0 C. or below when combined with a metal salt, and contain at least two polymers, the first of which is an ionically conductive polymer. In preferred embodiments, the monomers of this ionically conductive polymer have a hetero atom with a lone pair of electrons available for the metal ions of a metal salt to attach to and move between during conduction, when the first polymer is mixed with a metal salt. The second polymer is preferably more inert with respect to ionic conductivity when combined with the metal salt and has greater strength than the first polymer, when each of the polymers is in the form of a thin film. In some embodiments the first polymer is chosen from the group consisting of linear polymers, random copolymers, block copolymers, comb-branched block copolymers, network structures, single ion conductors, polyvinylidene fluoride or chloride and copolymers of their derivatives, poly(chlorotrifluoroethylene), poly(ethylene-chlorotrifluoro-ethylene), and poly(fluorinated ethylene-propylene). The first polymer material may be polyethylene oxide (PEO), oxymethylene linked PEO, PEO-PPO-PEO crosslinked with trifunctional urethane; poly(bis(methoxy-ethoxy-ethoxide))-phosphazene (MEEP); a triol-type PEO crosslinked with difunctional urethane, poly((oligo)oxyethylene)methacrylate-co-alkali metal methacrylate; polyacrylonitrile (PAN), polymethylmethacrylate (PMMA), polymethylacrylonitrile (PMAN); polysiloxanes and their copolymers and derivatives, polyvinylidene fluoride or chloride and copolymers of their derivatives, poly(chlorotrifluoro-ethylene), poly(ethylene-chlorotrifluoroethylene), poly(fluorinated ethylene-propylene), acrylate-based polymer, or another solvent-free polymer or combination of the above polymers either condensed or crosslinked to form a different polymer or mixed physically. The polymer selected as the first polymer is combinable with a lithium salt such that the ionic conductivity of that polymer is enhanced.
In some embodiments of the base polymer material, the second polymer is chosen from the group consisting of polyester (PET), polypropylene (PP), polyethylene napthalate (PEN), polyvinylidene fluoride (PVDF), polycarbonate (PC), polyphenylene sulfide (PPS), polytetrafluoroethylene (PTFE), and other polymer materials that possess stability and strength characteristics similar to one of said group of polymers. The composition of the base polymer material may comprise about 1% to 99% (by weight of said base polymer material) of the first polymer with the remainder being at least one other second polymer.
Methods of manufacturing the new polymer electrolytes are also provided described herein. One method of making a solid polymer electrolyte having a conductivity greater than 1xc3x9710xe2x88x924 S/cm at 25xc2x0 C. or below comprises mixing together a base polymer material containing at least one ionically conductive polymer. The base polymer or polymer blend has an initial conductivity of at least about 1xc3x9710xe2x88x925 S/cm at 25xc2x0 C. when combined with a metal salt; a metal salt; an inorganic filler having an average particle size less than 0.05 micron in diameter and a surface area of at least about 100 m2/g; an ion conducting material having an average particle size less than 0.1 micron in diameter and an initial ionic conductivity of at least 2xc3x9710xe2x88x923 S/cm at 25xc2x0 C. The mixture may also include a liquid organic solvent for dissolving the polymer and salt, if necessary, depending on the choice of polymer, for instance. Also, a curing agent may be included in the mixture if a curable polymer is selected. The process may optionally include maintaining the temperature of the mixture at the melting point of the base polymer material. The process includes forming the mixture into a solid polymer electrolyte and, optionally, evaporating the liquid organic solvent if one has been employed in the mixture. Curing of the solid polymer electrolyte is another option, which may be included if a curable polymer has been employed. The process may include forming the mixture into a 0.2 to 100 microns thick solid polymer electrolyte film, preferably 0.2 to 10 microns thick, and more preferably 0.2 to 3 microns thick.
In certain embodiments of the method of making a solid polymer electrolyte the process includes preparing a polymer/salt intermediate composition containing said base polymer material and said metal salt. A portion or aliquot of the intermediate composition is then formed into an initial polymer/salt electrolyte and the conductivity is then determined. By determining a conductivity of at least 5xc3x9710xe2x88x925 S/cm at 25xc2x0 C. for this initial polymer/salt electrolyte, the selection of a suitable polymer or polymer blend as the base polymer material for the solid polymer electrolyte is ensured.
In certain embodiments the method of preparing the solid polymer electrolyte also includes preparing a polymer/salt/filler intermediate containing the above-described polymer/salt intermediate composition and an inorganic filler having an average particle size less than 0.05 micron in diameter and a surface area of at least about 100 m2/g. After forming an aliquot of this polymer/salt/filler intermediate into a solid, the conductivity is determined. Determining a conductivity of at least 1xc3x9710xe2x88x924 S/cm at 25xc2x0 C. for an polymer/salt/filler intermediate offers the user a way to advantageously choose more desirable polymer/salt and filler compositions for a particular application.
Certain methods of making a solid polymer electrolyte include stamping the polymer/salt/filler/ionic conductor mixture onto a substrate. In certain other methods the process of making a solid polymer electrolyte includes adding a liquid organic solvent to the mixture, and evaporating the liquid organic solvent prior to optionally curing of the solid polymer electrolyte. In some of these solvent-based casting or coating methods, the process of forming the mixture into a solid polymer electrolyte may include employment of any of a variety of methods, including knife coaters, doctor blade coaters, wire-wound bar coaters (Mayer rods), air knife (air doctor) coaters, squeeze roll (kiss coaters), gravure coaters, reverse roll coaters, cast film coaters and transfer roll coaters. In preferred embodiments of these methods the solid polymer electrolyte has a final thickness of 0.2 to 100 microns.
A preferred embodiment of the method of making a solid polymer electrolyte includes fixing together about 30 to 95% (by weight of solid polymer electrolyte) base polymer material comprising at least one ionically conductive polymer and having an initial conductivity of at least about 10xe2x88x925 S/cm at 25xc2x0 C. when combined with a metal salt; about 1 to 25% (by weight of the solid polymer electrolyte) metal salt; about 0.1-60% (by volume of the solid polymer electrolyte) inorganic filler having an average particle size less than 0.05 micron in diameter and a surface area of at least about 100 m2/g; about 0.1-80% (by volume of the solid polymer electrolyte) ion conducting material having an average particle size less than 0.1 micron in diameter and an initial ionic conductivity of at least 2xc3x9710xe2x88x923 S/cm at 25xc2x0 C. Depending on the base polymer material selected, the user may want to include a liquid organic solvent and a curing agent in the mixture.
Certain embodiments of the above-described methods omit liquid organic solvent in the mixture and maintain the temperature of the mixture at the melting point of the base polymer material. In this case, the solid polymer electrolyte may be formed by hot-melt resin casting to provide a final film thickness of about 2 to 4 microns and a final film width of about 20 to 30 inches. The solid polymer electrolyte may alternatively be formed by a hot-melt resin extrusion process, producing a final film thickness of about 2 to 25 microns. A polymer electrolyte membrane prepared by this method contains no liquid organic solvent and is characterized by its dry, non-tacky consistency and pin-hold free structure. Preferably this electrolyte employs a plasticizer salt and the final polymer electrolyte membrane is flexible. Preferably the membrane is also conformable so that it may be readily shaped or fitted into to the desired shape. In some embodiments of the manufacturing methods the polymer electrolyte mixture is hot-pressed or is subjected to hot-isostatic pressing to form the final electrolyte.
In some embodiments of the methods, the inorganic filler may be blended with the other ingredients of the mixture during extrusion of the mixture, and the preferred inorganic filler material is fumed silica or alumina.
In an alternative embodiment of a method of making a solid polymer electrolyte the process includes mixing together about 30-95% (by weight of solid polymer electrolyte) base polymer material comprising at least one ionically conductive polymer and having an initial conductivity of at least about 10xe2x88x925 S/cm at 25xc2x0 C. when combined with a metal salt; about 1 to 25% (by weight of solid polymer electrolyte) metal salt; about 0.1-20% (by volume of solid polymer electrolyte) inorganic filler having an average particle size less than or equal to 0.01 micron in diameter and a surface area of at least about 100 m2/g; about 0.1-80% (by volume of solid polymer electrolyte) ionic conducting material having an average particle size less than 0.1 micron in diameter and an initial ionic conductivity of at least 2xc3x9710xe2x88x923 S/cm at 25xc2x0 C. Optionally, a liquid organic solvent, and optionally, a curing agent, to form a mixture. An anhydrous liquid organic solvent may be used in certain embodiments of the method. In some variations of the solvent-based methods, the step of forming the solid polymer electrolyte includes casting the mixture on an inert substrate and evaporating a solvent. Preferably the polymer electrolyte is about 0.5 to 25 microns in thickness in its final form. In some embodiments the method may also include curing the solid polymer electrolyte after liquid casting, and in some embodiments the curing is accomplished by applying ultraviolet or electron beam radiation to the electrolyte.
In certain preferred embodiments of a method of making a solid polymer electrolyte, about 30-95% base polymer, about 1-25% lithium salt, about 0.1-20% inorganic filler and about 0.1-80% sulfide glass or ceramic Li ion conducting material are included in the mixture, along with liquid organic solvent and a curing agent, if necessary. Another preferred method employs a mixture similar to that described above but containing a plasticizer lithium salt, about 0.1-60% inorganic filler and about 0.1-80% glassy conductor or ceramic lithium conductor, together with an anhydrous organic solvent. The electrolyte is formed by casting the mixture into a sheet film and then evaporating the organic solvent. A curing step is included if the polymer or polymer blend requires curing. In this way a final electrolyte film thickness of less than 1 micron may be formed. Certain methods of making a solid polymer electrolyte are similar to that described above but substitute spraying, or atomizing, of the mixture instead of solvent casting to form the electrolyte on a substrate. The final electrolyte film thickness may be about 0.2 to 3 microns.
A preferred embodiment of the invention provides a method of making a solid polymer electrolyte for a lithium polymer electrolyte battery. This method includes mixing about 30 to 95% (by weight of solid polymer electrolyte) of the base polymer material described above, about 1 to 25% (by weight of solid polymer electrolyte) plasticizer lithium salt, about 0.1-60% (by volume of solid polymer electrolyte) of an inorganic filler having an average particle size less than 0.05 micron in diameter and a surface area of at least about 100 m2/g, and about 0.1-80% (by volume of solid polymer electrolyte) lithium ion conducting material chosen from the group consisting of glassy conductors and ceramic lithium ion conductors, to form a polymer/salt/inorganic filler/ion conductor mixture. The mixture is then cast or extruded to yield a solid polymer electrolyte in the form of a film having a conductivity greater than 1xc3x9710xe2x88x924 S/cm at 25xc2x0 C. or below. In some embodiments of this method the temperature of the mixture is maintained at the melting point of the base polymer material, and in some embodiments the inorganic filler is fumed silica or alumina. In a preferred embodiment of this method a similar mixture is prepared using an ion conductor having an average particle size less than or equal to 0.01 micron in diameter.
In certain embodiments of the methods, very thin film forms of the polymer electrolytes are made at high speed using an automated technique comprising extrusion of the polymer electrolyte. One advantage of this type of method is its low cost compared to conventional polymer electrolyte manufacturing methods. An automated resin melt-cast process for manufacturing a thin film solid polymer electrolyte includes preparing a polymer/salt/inorganic filler/ionic conductor mixture as described above, and while maintaining the temperature of the mixture at the melting temperature of the base polymer material, the mixture is mechanically extruded onto a chilling wheel to yield a melt-cast film. By mechanically pulling the melt-cast film at predetermined speed, tension and heating conditions, the cast film is stretched to a final film thickness of about 4 microns or less to produce a sheet film solid polymer electrolyte having a conductivity of at least 10xe2x88x924 to 10xe2x88x923 S/cm at 25xc2x0 C. when combined with a plasticizer lithium salt.
In an alternative embodiment of the automated manufacturing methods, a polymer/salt/filler/ionic conductor mixture prepared as described above is sprayed onto a mechanically operated high speed moving substrate and the solvent is then evaporated. The polymer electrolyte is then cured, if necessary, depending on the particular polymer that is selected.
Still further provided by the present invention is an improved process for making a thin lithium polymer electrolyte rechargeable battery having an anode, a cathode and a polymer electrolyte. A preferred improvement comprises substituting for a conventional anode and current collectors an ultra thin film metallized polymer substrate having a thickness of about 0.5 to 50 microns and a lithium metal layer about 0.1-100 microns thick overlying a metallized layer of the metallized polymer substrate. This improvement also includes substituting for a conventional cathode and current collectors an ultra thin film metallized polymer substrate having a thickness of about 0.5 to 50 microns and an active cathode material layer about 0.1-100 microns thick overlying the metallized layer of a metallized polymer substrate. A further aspect of the improvement includes substituting one of the thin film solid polymer electrolytes described above for the conventional polymer electrolyte. In the preferred versions the metal layers of the anode and cathode metallized polymer substrates are up to 1 micron thick, more preferably about 0.01 micron thick.
Also provided by the present invention is a thin electrode for a lithium polymer electrolyte battery. In certain preferred embodiments an ultra-thin film metal substrate is employed for the cathode substrate or the anode substrate, or for both. The preferred thickness of the ultra-thin film metal substrate is about 1 to 10 microns. A layer of active electrode material overlies one side of the metal substrate and a layer of solid polymer electrolyte overlies the active material. In other embodiments the electrode substrate is a metallized polymer substrate about 1-10xcexc thick and comprising a polymer layer and a metallization layer having a resistance of about 0.01-1.0 ohm per square. The metallization layer adheres to one side of the polymer layer. Preferably the metallized polymer substrate includes a non-metallized margin, which may be about 1 to 3 mm, extending from an edge of the polymer layer to an edge of the metallization layer. Certain of these substrates also have a second metallization layer adhered to the other or opposite side of the polymer layer. The second metallization layer also extends about 1 to 3 mm from the same edge of the polymer layer to an edge of the second metallization layer. A layer of another electrode material may overlie the second metallization layer.
Further provided by the present invention is a solid state laminar electrochemical cell comprising an anode layer; a cathode layer; a layer of a solid polymer electrolyte, as described above, disposed between the anode and cathode layers. A current collector is attached to each anode and cathode, respectively. In certain preferred embodiments the current collectors are a very thin material such as a 5xcexc or less thick metallic element, or a 0.5 to 50xcexc thick metallized plastic.
The present invention also provides an orientation tolerant polymer electrolyte battery that has an all-solid-state composition. Orientation tolerant means that the operation of the battery is unaffected by its position. By contrast, a conventional liquid solvent polymer battery standing upright will tend to have the liquid solvents travel to the bottom of the cell, and during charge and discharge the current along the cell height will vary because of the difference in the conductivity at the bottom of the electrode and at the top. Such orientation sensitive cells typically do not to have very high cycle life and lose capacity as a result of poor charging and discharging. The orientation tolerant polymer electrolyte batteries of the present invention do not suffer from such drawbacks, as they include an all-solid-state electrochemical cell as described above.
A thin film lithium polymer electrolyte battery is also provided by the present invention. Certain embodiments of this battery comprise a resilient flexible hybrid polymeric electrolyte thin film that includes a homogeneous blend of at least two polymers with inorganic filler dispersed therein, impregnated with a lithium ion conducting glass finely dispersed therein and lithium salt; and a pair of spaced-apart flexible thin film electrodes, each including a polymer substrate having an adherent electrically conductive layer thereon, the hybrid film being tightly disposed, or sandwiched, between the pair of thin film electrodes. The polymer substrate of each of the anode and cathode is preferably selected from a group of polymers including PET, PP, PPS, PEN, PVDF and PE, and each polymer substrate is preferably a metallized polymer substrate with a thin metal layer as the adherent conductive layer. The metallized polymer substrate may be about 0.5 to 50 microns thick, thereby rendering it very flexible for ease of coating and handling, to avoid kinking and deformation of the substrate during manufacture of lithium polymer electrolyte batteries. Preferably there is a low resistance metallization layer having a conductivity in a range from about 0.01-1 ohm per square overlying and adhered to a side of the polymer material. Preferably, the layer of polymer material has a non-metallized margin with a width in the range from about one mm to about three mm extending from an edge of the substrate to an edge of the metallization layer. Also preferably a low resistance metallization layer having a conductivity in the aforementioned range overlies and adheres to each respective side of the polymer material, and both sides of the layer of polymer material have such a non-metallized margin present at the same edge of the layer of polymer material.
Still further provided by the present invention is an improved polymer electrolyte battery which contains at least one polymer electrolyte layer. The improvement includes substitution of one of the above-described solid state polymer electrolytes for the conventional polymer electrolyte layers of the battery. By employing the new solid state polymer electrolytes in a polymer electrolyte battery instead of a conventional polymer electrolyte, a cell resistance comparable (i.e., less than or equal) to that of a liquid electrolyte battery is obtained.
Such an improvement is particularly advantageous for providing an improved rechargeable lithium polymer electrolyte battery. A solid state lithium polymer electrolyte, as described above, characterized by its overcharge resistance, may be substituted for a standard lithium polymer electrolyte, which typically includes an organic liquid. Further improvement is obtained by substitution of ultra-thin current collectors for the conventional current collectors. Preferably the improved current collectors are chosen from the group consisting of metallic elements xe2x89xa65xcexc thick and metallized plastics 0.5 to 25xcexc thick. Such improved lithium polymer electrolyte batteries offer many advantages over conventional lithium secondary batteries, including improved energy density, power density, higher capacity utilization, higher cycle life, greater charge-discharge efficiencies, lower polarization, lower self-discharge, wider operating temperature range, and greater safety and reliability. The improved lithium rechargeable batteries can be produced at high speed, lower cost and with improved flexibility and conformability.
According to yet another aspect of the invention, an improved method of fabricating a thin lithium polymer electrolyte rechargeable battery includes incorporating an ultra thin metallized polymer substrate in the battery during fabrication in place of a conventional electrode substrate. The polymer layer of the new thinner substrate has a thickness in a range from about 0.5 micron to about 50 microns. The thickness of the metallization layer on the polymer layer is selected according to the desired conductivity, and is preferably less than about 0.01 micron. The method also includes substituting very thin film battery electrode/electrolyte structures, each 5 microns or less in thickness, in place of their conventional battery counterparts. The improved battery also substitutes a new solid polymer electrolyte, in accordance with one aspect of the invention, for a conventional polymer electrolyte.
These and other embodiments, objects, features and advantages of the present invention will become apparent with reference to the following description and drawings.